It is well known that a low-carbon (high-strength) and low-alloy steel is one of the most important engineering structural materials, and is widely applied to petroleum and natural gas pipelines, ocean platforms, shipbuilding, bridges, pressure vessels, building structures, automobile industry, railway transportation and machine manufacturing. The performance of the low-carbon (high-strength) and low-alloy steel depends on the chemical components and the process system in the manufacturing process thereof, wherein the strength, toughness and weldability are the most important performances of the low-carbon (high-strength) and low-alloy steel, and it is eventually determined by the micro-structure state of the finished steel product. As science and technology is continuously developing forward, people propose higher requirements for the strength-toughness and weldability of the steel, i.e. greatly improving the performance of the steel plate while maintaining relatively low manufacturing costs, so as to decrease the usage amount of the steel and save costs, reduce its own weight of the steel structure, and improve the safety of the structure.
Since the end of the 20th century to now, a research climax of developing a next generation of steel materials is aroused worldwide, which requires obtaining a better structure matching through optimizing the alloy combination design and renovating the TMCP process technique, without any increase in the contents of noble alloy elements such as Ni, Cr, Mo and Cu, etc., thereby obtaining a higher strength-toughness, a better weldability, and the adaptation of welded joints to the spraying method with various metals of Al and Zn etc.
When manufacturing a thick steel plate having a yield strength of ≥415 MPa and a low-temperature impact toughness at −60° C. of ≥34 J in the prior art, a certain amount of Ni or Cu+Ni elements (≥0.30%) are generally added, for example [The Firth (1986) international Symposium and Exhibit on Offshore Mechanics and Arctic Engineering, 1986, Tokyo, Japan, 354; “DEVELOPMENTS IN MATERIALS FOR ARCTIC OFFSHORE STRUCTURES”; “Structural Steel Plates for Arctic Use Produced by Multipurpose Accelerated Cooling System” (Japanese), Kawaseki Seitetsu Gihou, 1985, No. 1 68-72; “Application of Accelerated Cooling For Producing 360 MPa Yield Strength Steel plates of up to 150 mm in Thickness with Low Carbon Equivalent”, Accelerated Cooling Rolled Steel, 1986, 209-219; “High Strength Steel Plates For Ice-Breaking Vessels Produced by Thermo-Mechanical Control Process”, Accelerated Cooling Rolled Steel, 1986, 249-260; “420 MPa Yield Strength Steel Plate with Superior Fracture Toughness for Arctic Offshore Structures”, Kawasaki steel technical report, 1999, No. 40, 56; “420 MPa and 500 MPa Yield Strength Steel Plate with High HAZ toughness Produced by TMCP for Offshore Structure”, Kawasaki steel technical report, 1993, No. 29, 54; “Toughness Improvement in Bainite Structure by Thermo-Mechanical Control Process” (Japanese), Sumitomo Metal, Vol. 50, No. 1 (1998), 26; “Structural Steel Plates for Ocean Platform used in Frozen Sea Areas” (Japanese), Research on Iron and Steel, 1984, No. 314, 19-43], so as to ensure that the steel plate as the base material has an excellent low-temperature toughness, the toughness of the heat-affected zone HAZ also can reach Akv34 J at −60° C. when welding with a heat input of <100 KJ/cm; however, the steel plate does not involve a resistance to zinc-induced crack.
The above-mentioned large number of patent documents only demonstrate how to achieve the low-temperature toughness of the steel plate as the base material, and explain less about how to obtain the excellent low-temperature toughness of the heat-affected zone (HAZ) under a welding condition, and even do not relate to how to ensure that the structure of the heat-affected zone is homogeneous and tiny ferrite+a small amount of pearlite especially when welding using a high heat input, enable the ferrite to nucleate and grow on the prior austenite grain boundary, substantially eliminate the prior austenite grain boundary, and improve the resistance to zinc-induced crack of the steel plate, such as Japan patents S 63-93845, S 63-79921, S 60-258410, Published Patent H 4-285119, Published Patent H 4-308035, H 3-264614, H 2-250917, H 4-143246 and U.S. Pat. No. 4,855,106, U.S. Pat. No. 5,183,198, U.S. Pat. No. 4,137,104 etc.
At present, only Nippon Steel Corporation adopts an oxide metallurgical technology for improving the low-temperature toughness of the heat-affected zone (HAZ) when using a high heat input welding for the steel plate, and this patent also does not involve how to improve the zinc-induced-crack-resistance of the steel plate, see U.S. Pat. No. 4,629,505 and WO 01/59167A1.